The present invention relates to an aqueous alkaline flood process for recovering oil by injecting an aqueous alkaline solution into a subterranean oil reservoir containing an acidic oil. More particularly, the present invention relates to such a process in which decreasing proportions of preformed cosurfactant material are included in the injected solution in a manner which solves a problem unique to aqueous alkaline flood processes.
Numerous aqueous alkaline flood processes have been proposed, and various processes involving injecting an aqueous alkaline solution and various preformed surfactants have been described in U.S. patents, such as the following: U.S. Pat. No. 3,771,817 describes injecting an aqueous alkaline solution to satisfy the surfactant adsorption sites on the reservoir rock and then injecting a surfactant-containing aqueous liquid which may also contain alkali. U.S. Pat. Nos. 3,804,170; 3,804,171 and 3,847,823 describe injecting aqueous alkaline solutions containing overbased petroleum sulfonate surfactants which are formed by over-neutralizing petroleum hydrocarbon sulfonates. U.S. Pat. Nos. 3,997,470 and 4,004,638 describe injecting an aqueous alkaline solution followed by an aqueous alkaline solution which contains a preformed surfactant which can be substantially any hydrocarbon sulfonate and can be accompanied by polyphosphates and carbonates that enhance the oil displacing efficiency of the process.
U.S. Pat. No. 4,099,569 describes a staged process for recovering oil from a subterranean reservoir by injecting a surfactant solution in which the concentration of the surfactant is increased as increasing amounts of the solution are injected and then injecting a drive fluid. U.S. Pat. No. 4,232,737 describes a staged injection of a highly saline aqueous petroleum sulfonate surfactant system containing a solubilizing amount of cosurfactant and decreasing the concentration of both the salt and surfactant in stages to provide a trailing-edge salinity which is suitable for a polymer thickened aqueous drive fluid.
Commonly assigned U.S. patent application Ser. No. 411,779, filed Aug. 26, 1982 by D. R. Thigpen, J. B. Lawson and R. C. Nelson (i.e., the "'779 application") relates to recovering oil from an acidic oil reservoir by injecting an alkaline aqueous solution. In the process of the '779 application, the alkaline solution also contains a substantially neutral salt and a preformed cosurfactant. That cosurfactant comprises at least one compound which is significantly soluble in both the aqueous alkaline solution and the reservoir oil while being more soluble in the aqueous solution (relative to its solubility in the reservoir oil) than are the petroleum acid soaps which can be formed from the reservoir oil. The cosurfactant solution is selected and its concentration is adjusted so that the injected solution has an alkalinity, salinity and preformed cosurfactant content such that the salinity of the surfactant system formed by the interaction of the injected solution and the reservoir oil is substantially optimum for minimizing interfacial tension between the oil and surfactant system.
The disclosures of the '779 application are incorporated herein by reference.
As indicated in the '779 application, although prior processes in which preformed surfactants were included in injected aqueous liquid solutions were designed to improve the oil recovery efficiency of similar processes free of the preformed surfactants, a serious problem remained in either type of such prior processes. Whenever an aqueous alkaline solution is injected into an oil reservoir, some or all of the alkali may be consumed by chemical reactions other than the desired reaction of converting petroleum acids to surfactant soaps. For example, multivalent cations dissolved in the water in the reservoir and/or associated with clay or other reservoir rock materials can rapidly consume alkali by forming and precipitating multivalent metal hydroxides or salts. In siliceous reservoirs significant proportions of alkali are consumed by dissolving silicon oxide and by forming alkali metal silicates, etc. Because of such side reactions, if the injected aqueous alkaline solution is dilute, the alkali will propagate slowly through the reservoir rocks. The frontal propagation rate is slow because, as each portion of the injected solution contacts fresh portions of rock, some or all of its alkali content may be consumed by the side reactions. This is repeated over and over, and thus, although the unreactive liquid components of the injected solution may move through the reservoir at the rate corresponding to the rate at which the solution was injected, the movement through the reservoir of the alkali may be much slower. For example, it is disclosed in SPE Paper No. 8995 by Bunge et al. that, when an aqueous alkaline solution containing 0.44% sodium hydroxide and 1.0% sodium chloride was flowed through a core of Wilmington sand which initially contained 1.0% calcium chloride solution; more than two pore volumes of the aqueous alkaline solution had to be injected before any of the sodium hydroxide reached the outflow end of the core.
But it is known that, for example, as indicated in U.S. Pat. No. 3,927,716, when an aqueous alkaline solution reacts with an acidic oil, the lowest interfacial tension between the aqueous solution and the oil frequently occurs when the concentrations of the alkali and neutral salt in the aqueous alkaline solution are low and are within a rather narrow range, such as about 0.01 to 0.04% by weight of alkali and 0.5 to 2.0% netural salt. This was the problem to which the '779 application was directed. The teachings of the prior art had not disclosed how to obtain the low interfacial tension required for a good oil recovery while injecting an aqueous alkaline solution containing the high alkali concentration required for a satisfactory rate of alkali propagation within the reservoir.
Although an inclusion of a cosurfactant as described in the '779 patent application tends to adjust the salinity and cosurfactant concentration of the injected aqueous alkaline fluid to a value capable of providing an optimum salinity in the surfactant system formed by the reaction of the injected fluid with the reservoir oil, a problem still remains. The operator of the oil recovery process has no control over the concentration of the primary surfactant. At any point in the process the concentration of primary surfactant is determined by the oil-water ratio and the oil-water ratio is variable. One especially critical region is the flood front. There the oil saturation is high, but the injected slug has been diluted by the formation water. The dilution causes both the alkaline material and cosurfactant concentrations to be below their injected values. Therefore, if the alkaline slug is designed to be optimum for the residual oil-water ratio, the slug is apt to be over-optimum at the flood front. If, on the other hand, the slug is designed to be optimum at the flood front, it is apt to be under-optimum behind the flood front where oil saturations are lower.